We have demonstrated that uPA is required for effective pulmonary defense against Cryptococcus neoformans (C. neo) infection in vivo. Host defense against C. neo is absolutely T cell dependent. In the absence of uPA, lung accumulation of CD4 cells fails to occur in response to infection, and the uPA minus/minus animals die from disseminated Cryptococcosis. We found that the T cell impairment in the absence of uPA is not specific to C. neo. In vivo, uPA minus/minus mice fail to increase pulmonary CD4 cells in response to P. carinii, and do not clear the infection. We have found that T cell proliferative responses are profoundly impaired in the absence of uPA. In vitro, uPA minus/minus lymphocytes do not undergo antigen-specific proliferation, nor do they proliferate in response to TCR- mediated signals generated by CD3-cross-linking, or by mitogen. Effective host defense to T-cell specific pathogens in vivo; and normal responses in vitro T cell proliferation assays induced by antigen, mitogen, or by CD3-cross-linking, are linked by a common signaling pathway mediated primarily through the TCR-protein kinase cascade, and coupled to co-stimulatory signals delivered by CD28 and CD25. It is our hypothesis that in the absence of uPA, T lymphocyte intracellular signaling via Jak/STAT is diminished leading to inadequate cytokine production and diminished cytokine responsiveness. The lack of Jak/STAT signaling results in blocked T cell activation and proliferation; and T cell unresponsiveness to further TCR-mediated signaling. 1. Localize the biochemical block(s) in T cell signaling cascades. a) Determine if the absence of uPA diminishes TCR- mediated signaling. b) Determine if the absence of uPA diminishes CD28- and CD25-mediated signaling. c) Determine whether uPA activates Jak/STAT signaling in murine T lymphocytes. 2. Determine the functional consequences of aberrant T cell signaling. a) Determine the requirement for uPA in lymphocyte activation as measured by expression of activation antigens, elaboration of cytokines, and responsiveness to IL-12. b) Determine whether the defects in uPA minus/minus lymphocyte signaling result in anergy, cell cycle arrest, or apoptosis. 3. Determine the cellular expression and trafficking of uPA and uPAR during T cell activation and proliferation in vitro. Determine if recombinant uPA can reverse the functional abnormalities in uPA minus/minus lymphocytes.